Actinomycetes (particularly thermophilic species) have long been considered as major components of microflora of compost which are colonized by the white button mushroom (Agaricus bisporus). They have the ability to degrade complex molecules such as cellulose, lignocellulose and lignin. These micro-organisms have also the ability to considerably produce degrading enzymes and antibiotics. In addition, microbial biomass of actinomycetes contains large amounts of nitrogen and mineral nutrients required for mycelia growth of the white button mushroom and its development. In this study, 105 isolates of thermophilic actinomycetes were collected from phase-II compost of A. bisporus from various Iranian mushroom farms, using serial dilutions followed by microbial culture purification. The isolates were then identified and characterized by morphological and biochemical tests followed by pattern analysis of digested 16S rRNA gene. The isolates were then screened for their cellulolytic activity on carboxymethyl cellulose at 45oC and 50oC and at different pH values (6.5, 7, 7.5, and 8). The selected isolates were used for preparing culture medium of A. bisporus based on microbial biomass. The selected actinomycetes isolates were also categorized in five groups and ten genera of which 84 isolates belonged to Group II and the genera Nocardiopsis, Actinomadura, Thermobiospora and Thermomonospora. The results of cellulase activity revealed that optimum cellulolytic activity of actinomycetes was at 50oC and neutral pH. Both inoculation of compost with the selected actinomycetes and use of bacterial biomass as culture media enhanced growth of mycelia of A. bisporus in comparison to the control. Finally, thirteen isolates of actinomycetes were considered promising candidates for compost artificial inoculation based on the conditions of temperature, pH of phase-II compost, cellulolytic activity and the effect of microbial biomass on growth of mushroom mycelia.